Currently, controls that have popups can have their z-index controlled as explained here: http://www.telerik.com/help/aspnet-ajax/controlling-absolute-positioning-with-zindex.html RadComboBox has an explicit ZIndex property. Should all controls with popup elements have such a property?
I will mark this feature request as 'Completed' since the list with components that offer ZIndex property is now bigger. You can find them enlisted here - http://www.telerik.com/help/aspnet-ajax/controlling-absolute-positioning-with-zindex.html. If you want a ZIndex property for another control, just drop us a line below.
I think this should be added to controls so that a ZIndex property could be set on a control rather than putting additional attributes on a server tag that are not part of it and get passed through.